The common factor between the night tube lines are that they are operated under ATO (bar the Piccadilly) and so there is less chance of driver error at night (fatigue, light conditions, &c.), because the train is being driven by itself. The Piccadilly, whilst not ATO, is on the map primarily because it serves Heathrow. The SSL isn't on the map currently because it has got an impending upgrade to fit ATO equipment, which will result in weekend shutdowns and night time works, and so would require it to be removed from the map and then re-added later. Once it has gone ATO, it will likely go onto the map.